1. Field of the Invention
The present invention relates to an image processing technique for performing gradation conversion of an image.
2. Description of the Related Art
Before an image radiographed by a radiographic device, such as an image sensor or a camera, is displayed on a monitor or in a film for X-ray diagnosis, the image data is generally subjected to gradation conversion so that the image can be observed more easily.
For example, in a so-called two-point method, gradation conversion is performed such that a maximum pixel value and a minimum pixel value in an area subjected to gradation conversion processing (i.e., an object area) are set to constant densities. According to this method, the entire object area is displayed at a constant density range, so that the entire object area can be displayed with high reliability.
In addition, in a so-called one-point method, gradation conversion of, for example, an image of a chest radiographed from the front is performed such that a maximum pixel value in an area of the image corresponding to a lung depicted in the image is set to a constant density. More specifically, in the one-point method, gradation conversion is performed such that a pixel value in a target region in the object area, which is also called a characteristic region and refers to a specific anatomic region, is set to a predetermined density. According to this method, since the density in the target region is set to a predetermined density, diagnosis of the target region can be performed with high reliability.
On the other hand, a method of changing a dynamic range has been proposed by the inventors of the present invention (Japanese Unexamined Patent Application Publication No. 10-272283). More specifically, an image processing method according to this publication can be expressed by using coordinates (x, y) in an image, a gradation conversion function F1( ), a gradation conversion rate c(x, y), a pixel value fd(x, y) after the conversion, a first image f0(x, y), a second image f1(x, y), and a smoothed (low-frequency) image fus(x, y) of the second image, as follows:fd(x,y)=f0(x,y)+(1−c(x,y))×(f1(x,y))−fus(x,y)  Equation (1)where f0(x,y)=F1(f1(x,y)),
      c    ⁡          (              x        ,        y            )        =            ∂              F1        ⁡                  (                      f1            ⁡                          (                              x                ,                y                            )                                )                            ∂              f1        ⁡                  (                      x            ,            y                    )                    
In this method, the dynamic range of the image is changed while high-frequency components are adjusted, so that the entire object area can be displayed in a film with good contrast without reducing the amplitude of fine structures.
In the above-described two-point method, since gradation conversion is performed such that the maximum and the minimum pixel values in the object area are set to constant densities, the concept of setting the density in the target region in the image to a predetermined density is not applied. Accordingly, there is a problem in that the density in the target region varies with each image.
In addition, in the above-described one-point method, although the density in the target region is set to a predetermined density, there is a problem in that the maximum and the minimum densities in the object area vary with each image depending on the width of a pixel value distribution (pixel value width) in the object area. For example, when gradation conversion of an image of a chest radiographed from the front is performed such that the maximum pixel value in a lung field is set to a constant density, the density in an abdomen region, where pixel values are low, varies with each image depending on the pixel value width of the object area. In addition, although the peripheral region of the lung field is also necessary for diagnosis, since this region is adjacent to the abdomen region and the pixel values are also low, the density in this region also varies with each image. Although gradation conversion is not necessarily performed such that the maximum and the minimum pixel values in the object area are set to constant densities as in the two-point method, it is undesirable for the contrasts in regions corresponding to the maximum and/or the minimum pixel values in the object area to vary significantly with each image depending on the pixel value width of the object area.
In addition, the above-described method of changing the dynamic range does not serve to solve the above-described problems of the one-point method and the two-point method. More specifically, the density in the target region, as well as in other regions cannot be prevented from varying with each image.